The present invention relates to a fixing apparatus for dry electrostatic photography.
Thermal fixing apparatus comprising a thermal fixing roller to fix a toner image to a copy sheet is known in which an offset preventing liquid which is typically a silicone oil is applied to the surface of the fixing roller. In one known form a felt wick has one end immersed in a container of the oil and the other end contacting the fixing roller. In another known form an applicator roller contacts the fixing roller and is partly immersed in a container of the oil. A problem common to both of the systems is that the amount of liquid applied to the fixing roller cannot be readily controlled. After a period of operation the amount of liquid usually becomes excessive and contaminates the copy sheet and/or smears the toner image.
An improvement to such a fixing apparatus comprises a transfer roller, an applicator wick and a spring pressing the wick into contact with the transfer roller and the transfer roller into contact with the fixing roller. The spring force is selected in such a manner that the transfer roller is rotated by the fixing roller only when the thickness of liquid on the fixing roller drops below a predetermined value and thereby the coefficient of friction between the transfer roller and the fixing roller increases above a corresponding predetermined value to apply more liquid to the fixing roller.
A drawback to such a system is that it is not operable in a case in which the fixing roller is formed of a hard material such as TEFLON (TRADEMARK). The reason is that the coefficient of friction between the transfer roller and the fixing roller is always lower than that between the transfer roller and the wick, and the transfer roller is therefore not rotatable by the fixing roller. Increasing the spring force, although increasing the normal force between the transfer roller and the fixing roller also increases the normal force between the transfer roller and the wick, and is not effective in rendering the transfer roller rotatable. Reducing the spring force to a level close to zero renders the transfer roller rotatable but in an unstable manner which results in excessive liquid being applied to the fixing roller.